1. Field of the Invention
The present invention relates to a power supply system for a motherboard.
2. Description of Related Art
Power provided to a motherboard includes primary and secondary power. The motherboard uses a 24-pin ATX power connector to receive its primary power, and the primary power is regulated to provide secondary power according to different requirements of a plurality of chips on the motherboard. If the secondary power is not stable, for example, spikes to a high level, a north bridge or a south bridge may be burned out.
Referring to FIG. 1, a conventional 2.5V power supply system for a motherboard includes a 3.3V standby power source V11, a 3.3V system power source V12, a voltage divider 100, a linear adjuster 200, and a feedback resistor R13. The voltage divider 100 includes a resistor R11 and a resistor R12 connected in series. The 3.3V standby power source V11 is grounded via the voltage divider 100. The linear adjuster 200 includes an operational amplifier (OA) U11, and a N-channel Metal Oxide Semiconductor (NMOS) transistor Q11. A non-inverting input terminal of the OA U11 is connected to a node between the resistor R11 and the resistor R12. An inverting input terminal of the OA U11 is connected to a source of the NMOS transistor Q11 via the feedback resistor R13. An output terminal of the OA U11 is connected to a gate of the NMOS transistor Q11. The source of the NMOS transistor Q11 is grounded via a capacitor C11 for wave filtering. A drain of the NMOS transistor Q11 is connected to the 3.3V system power source V12.
Power from the 3.3V standby power source V11 is divided by the divider 100 so that a 2.5V voltage is delivered to the non-inverting input terminal of the OA U11. In the beginning, voltage at the inverting input terminal of the OA U11 is zero, so the output terminal of the OA U11 outputs a high level signal, then the NMOS transistor Q11 starts to turn on. The OA U11 acts as a short, so voltage at the inverting input terminal of the OA U11 is also 2.5V. Meanwhile, there is no current flowing through the non-inverting input terminal and the inverting input terminal of the OA U11. That is, there is no current flowing through the feedback resistor R13, so there is no voltage dropped across the resistor R13, so voltage at a node between the feedback resistor R13 and the source of the NMOS transistor Q11 is 2.5V, which is provided from the 3.3V system power source V12 via the NMOS transistor Q11. The 2.5V voltage is provided to a north bridge or a DDR1 memory of the motherboard. The voltage at the inverting input terminal of the OA U11 follows the voltage at the non-inverting input terminal of the OA U11 via the feedback resistor R13 to provide a stable voltage to a load.
Commonly, a short circuit develops between the drain and source of the NMOS transistor Q11 because of poor quality or workmanship. Thus, the power supply circuit loses its feedback function and a 3.3V output voltage is continuously provided to the 3.3V system power source V12. The 3.3V output voltage is far greater than an operating voltage of the north bridge or the DDR1 memory, which can destroy the north bridge or the DDR1 memory.
What is needed is a power supply circuit having an over-voltage protecting function for a motherboard.